Alternating-current dynamo-electric machine.



M. MILGH. ALTEBNATING CURRENT DYNAMO ELECTRIC MACHINE.

APPLICATION FILED APR. 8, 1904.

Patented Dec. 6, 1910.

S BHEETS-BHBET 1.

Inventor,

Witnesses M m.

pd; Atty.

M. MILGH. ALTEBNATING CURRENT DYNAMQ ELECTRIC MACHINE.

APPLICATION FILED APILB, 1904.

- Patented Dec. 6, 1910.

8 BHEBTS-BHEET 2.

Fig.5.

Mibneszs: Y T I Inventor.

Maurice Milch.

' Abby.

M. MILGH.

ALTBBNATING CURRENT DYNAMO ELECTRIC MACHINE.

APPLICATION FILED APR.8, 1904.

977,821 Patented Dc.6,1910.

3 BHEETB -BHEET 3.

Witnesses: Inventor".

Maurice Mi lch.

UNITED saAtr s PA ENT oF-FIoE.

MAURICE MILCH, 0F SCHENECTADY. NEW YORK, ASSIGNOR ToGENE RAL ELECTRIC COMPANY, A CORPORATION or NEW YORK.

ALTERNATING-CURRENT DYNAMO-ELECTBIC MACHINE.

12) all whom it may concern:

Be it known that I, MAURICE Minor-r, a subject of the King of Hungary, residing at Schenectady, county of Schenectady, State of New York, have invented certain new and useful Improvements in Alternating- Current Dynamo Electric Machines, of which the followin is a specification.

My invention re ates to alternating current dynamo electric machines of the commutator type having a fluctuating field, and its object is to improve the commutation of such machines. There are many types of alternating current motors known in the art, such as series motors, repulsion motors, and many modified forms, which depend for the principle of their operation on a commutator. It is much more difficult to secure sparkless commutation in an alternating current motor than in a direct current machlne, since in a direct current machine the coil, at the instant it is undergoing commutation, is in a constant field, while in an alternating current machine the coil which is short-circuited by the brush incloses a rapidly fluctuating field, which induces an electro-motive force.

and produces large current flow in the short circuited coil. This results in "heating and excessive s arking as the coil leaves the brush. It as been proposed heretofore to improve the commutation of alternating current machinesby reducing the current in the short circuited coils by means of resistances inserted in the commutator leads. The use of resistances, while it improves commutation, wastes ower. .It does not remove the cause of bat? commutation, butmerely 'reduces its effects.

By my invention I fully remove the cause of bad commutation in alternating current machinesby producing in each coil as it is shortci-rcuited by the brush an electro-motive force nearly equal and opposite in base to that induced by the fluctuating fiel In this manner I do not simply reduce the amount of the short-circuit currents, but neutralize the electro-motive force which produces them. a

My invention will best be understood from the accompanying drawings, in which Figure 1 is an explanatory diagram; Fig. 2 shows diagrammatically my invention applied to an alternating current commutator motor; Fig. 3 shows an arrangement for Specification of Iietters Patent.

Application filed April 8, 1904. Serial No. 202,134.

automatically varyin the current flow in the connnutating coi to meet therequirements of varying speed; Fig. 4 shows another arrangement for the same purpose; Fig. 5 shows an arrangement applicable for nolyphase motors; Fig. 6 shows a similar arrangement for single-phase motors; Fig. 7

Patented Dec. 6, 1910.

shows another arrangement for controlling the current. in the commutating coil and as applied to a repulsion motor; Fig. 8 shows an arrangement for compensating for the commutation reactance in addition to the fluctuating field effect produced by the motor currents; Fig. 9 shows an arrangement for utilizing the commutator coil to increase the starting torque of the motor; Fig. 10 shows a series motor arranged in accordance with my invention; and Fig. 11 is a detail of the stator punchings.

Referring first to Fig. 1, if F represents in magnitude and phase the fluctuating field threading a short-circuiting coil, E will represent the phase of the electro-motive force induced in thecoil by the field, lagging 90 degrees'behind the field. Now if a second electro-motive force E could be induced in the coil equal and opposite to E, the effect of the fluctuating field would be neutralized, and no short-circuited current would flow. In order to induce this second electro-motive force E, I take advantage of the motion of the short-circuited coil by producing a field F 90 degrees ahead of the field F, and arranged to be cut by the rotating shortcircuited coil. An electro-motive force IE will be induced in phase with the field F, and consequently o posite to the electro-lnotive force E. If tie field F is of the broper magnitude, the electro-motive force IE will be completely neutralized. I

In Fig. 2, S represents the stator and R the rotor of an alternating current motor of the commutator typel Neither the winding of the stator nor the connections of the stator or rotor are shown, since they are wholly immaterial for the purposes of my invention. Let it be merely assumed that a fluctuating field exists as shown by the opposite the short-circuited coil, and if it is energized by a current, it will produce a field F, as indicated by the arrow. If this field F is of the proper magnitude and phase, an electro-motive force may be produced in the short-circuited coil equal and 0 posite to that induced by the fluctuating eld 1. Thus the flow of current in the short-circuited coil will be prevented, and sparkless commutation obtained.

Fig. 3 shows one arrangement for obtaining the proper magnitude and hase for the current 111 the commntating 001 C. In this figure I have shown a winding V adapted to produce a field as indicated by arrow F in Fig. 2, and connected in a circuit a a. The motor connections. are not shown in the diagram, since they are for present considerations of no importance. The motor may be series, repulsion, or any other type of commutator motor. The winding WV may be on stator or rotor. The current in it may be impressed or induced. It is simply assumed that the current in winding W produces a fluctuating field threading the coil short-circuited by brush b. A resistance R is connected in series with winding \V, so as to be traversed by the current which produces the fluctuating field, and the commutating coil C is connected to variable portions of the resistance R. A s eed governor G on the shaft of the motorv shifts one of the points of connection of coil C along the resistance B, as the motor speed varies. With this arrangement a roper phase and magnitude of current in t e commutating coil ma be obtained, as will ap ear from the to lowing considerations. he current flowing through the resistance R has been assumed to be the current which produces the fluctuating field in the motor. The across the terminals of resistance is consequentl always in phase with the fluctuating fiel that is, in phase with the line F in Fig. 1. The impressing of this voltage upon the commntating coil causes a field displaced by nearly 90 degrees from the voltage, which mayl be represented by the line F in Fig. 1: t at is, the field produced by the commutating coil is of the proper phase. As regards ma nitude, the potential across the terminals 0 R is directly 'pro ortional to the current flowing in windin and as the fluctuating field is produced y the current flowing in winding W, the potential flowing across resistance R is approximately proportional to the fluctuatingfield. Consequently the field produced by the commutating coil 0 varies proportionally to the fluctuating field, and may be of the proper value for all values of current flow m the motor. The electro-motive force induced in the short-circuited coil, due to cuttin the field of coil C,'is, however, roportiona not only to the field strength o 0011 Q, butialso to the speed of rotation of the short-circuited c011. Thefunction of the governor G otential glesphase motor M, 'a proper is to vary the current throu h coil C, and consequently its field stren h as the speed of the motor varies. As t e motor speeds up, the potential across the terminals of coil C is automatically reduced by the governor G, and in this manner the proper counterpotential in the short-circuited coil is always maintained.

Fig. 4 shows another arrangement for automatically maintaining the proper current flow through the conimhtating coil C. In this arrangement a separate exciter D is employed for the commutating coil and the field of the exciter is connected in series with the winding \V, thereby insuring the proper phase relation at all times for the electromotive force impressed. upon the coil C, since the electro-motive force induced in the armature of exciter D mustat all times be in phase with the current in winding V.

Moreover, the value of the electro-motive V force impressed upon coil C will vary with the current through winding W in the same manner as in Fig. 3, since the electro-motive force induced in the armature of exciter D is roportional to the current in its field coils.

n order to obtain the proper variation of the impressed electro-motive force in coil C with the variation in speed of the motor, the differential gear H may be employed, one member [1. being driven by the motor, while a second member it may be driven at constant s eed from any convenient source of power.

he third member h" drives the armature of exciter D. Thus, when the motor is sta- .tionary, the armature of exciter D will rea e F 1g. Here a polyphase series transformer. is utilized for this urpose, the variation in the electro-motive orce im ressed upon the coils C being secured by shifting the points of connection of these coils to the pol phase transformer T. This shifting may obtained automatically by the s eed governor shown in Fig. 3, or by- .any 0t er equivalent arrangement. Fi 6- shows a r arrangement applica Is to single-phase motors. When no polyphase source is available, the equivalent of a polyphase transformer may be obtained by using a single-phase induction motor which has the characteristic of roducing a rotating field in its windings. shifting the points of connection of the 0011 C along the taps'on the primary windifi of the sinjustment of when the phase and magnitude of the electro-motive force impressed upon the coil '(i may be obtained.

In Fig. 7 I have shown mother arrangement for obtaining suitable phase and magnitude rotations of the electro-motive force impmssedupon the commutating coil, and have shown it a' lied to a repulsion motor. The stator win in S is connected to a voltage regulator as shown, while the electro-motive force tor the 'commu'tatin-g coils C is obtained from the variable secondary of a potential transformer cm-mected across the motor temi'inals. Therotor R is shown short-circuited on itself in the usual manner. Since the 'ten'tial of the secondary of transformer is practically 180 degrees cm of phase with the voltage impressed upon the primary member, and since the fluctuating field in a repulsion motor at normal speeds is approximately in phase with the impressed voltage, the electro-motive force impressed upon the'co ils C is approximately in phase, or 180 degrees out of phase, with the fluctuating field, and the currentin the coils C is oonsequentl approximately 90 de reesoutof phase wit this fluctuating fie'l that is, it is of the phase desired. The ratio of transformation of the potential transformer E may be varied by the speed governor of Fig. 3, or a similar device acting tn the movable contact Consequently the impressed voltage on te coils C ma be kept of the roper value for all spee' s at-any given va ue of impressed voltage on the motor. Furthermore, since the primary of potential transtormer P is connected across the motor terminals, the electro-motive force impressed on coil C will be of the proper amount for all values of the voltage impressed upon the motor, since the voltage impressed upon the co:ls C is proportional to the voltage impressed upon the *motor, which in turn is proportional to'the fluctuating field. Consequently 'the magnitude of the voltage impressed u n "the coil C will be correct for all loa s and for all values of impressed voltage on the motor. This arrangement for controlling the compensating coils is ap licable' not only'to a repulsion motorias s own, but also to any form of alternating current motor of the commutator type. In the arrangement of Fig. 7 the secondary of the potential transformer P is made not only variable, but also reversible. The reason for this is that in the repulsion motor a field exists at all times, which acts as a commntating field. The fluctuating field, which produces sparking in the short-circuited coils, is a field produced by the. working currents of the motor, and is'conseqnently, atnormal speeds, nearly inphasefwi'th the impressed voltage. There is a second field, however, which\pro'- 'tro-motive force.

duces the counter-electromotive force of the motor. This field is in phase with the magnetizing component of the primary current, and may be called the magnetizing field. It is 90 degrees outof phase with the impressed voltage. in other words, this second field is of approximately the proper phase to assist in producin proper commutation. Below synchronism t e electro-motive force produced in the short-circui-ted coils by the fluctuating held of the working currents is greater than that due to cutting the magnetizing field. As the speed of the motor increases, however, and as the amount of current flow in the motor decreases, the electro-motive force induced in the shortcircuited coil by cutting the magnetizing field increases, while the other electro-motive forces decrease, until above synchronism the cutting electro-motive force overpowers the electro-motive force due to the fluctuat ing field, and itself becomes the cause of heavy short-circuit currents and sparking.

In order to enable the motor to operate with field due to the magnetizing currents of the motor below synchronism, and to opposethat field above s nchronism. With this arrangement spark ess operation of the motor may be obtained through its entire range. The same is true of the arrangement of Fig. 4, since if shaft h is driven at synchronous speed the direction of rotation of exciter D will be reversed when the motor runs above synchronism.

When the transformer arrangement of Fi' 7 is used for controlling commutating 0015 of an alternatin current motor, the electro-motive force 1mpressed upon the commutating coils is of the proper phase relation only so long as the motor currents are nearly in phase with the impressed elec- At starting, the power factor of alternating current motors is low, and the motor current is more nearly degrees out of phase with the impressed vo tage than in phase with'it. The electromotive force impressed upon the coils C is, as has already been stated, 180 degrees out of phase with the impressed voltage. Consequently at starting the voltage impressedon coil 0 is little more than 90 degrees out of phase with the motor currents. The current in. coils O, which is 90 degrees behind the impressed voltage, will consequently be practically in phase with or in in commutation. At low speeds, however,

the question of commutation may be a less serious one if the current in the commutator coils be utilized to increase the starting torque of the motor; since, being nearly 180 degrees out of phase with the motor cur rents, it is of the proper phase relation for producing torque. This torque may be positive or negative, according to the connection of the commutating coils; and to produce a positive torque, the commutating coils should be connected to the secondary of transformer P oppositely to the way they are connected for o eration below synchronism. In other wor s, the movable contact p should be moved to one side of its central position at starting, in order to produce a torque which will assist the motor in starting; and then when the motor begins to revolve, it should be returned to the other side of its central position. This variation in the connection of the coils'G may be ob-.

tained independently of the transformer by the reversing switch, if preferred.

When the potential for the commutator coils is obtained from the terminals of a resistance connected in series with the working currents of the motor,the potential impressed upon these coils will always be of the proper phase relation for pro ucing a commutatin field; If, however, it is desired to uti ize the commutating coils for increasing the starting torque of the motor, the arrangement of Fig. 9 may be employed. In this arrangement, the switch 8 1s pro vided for shifting the terminal connections of the coil C from the resistance R to the impedance I, and'the switch 8 is rovided for reversing the connections. 0 coil C. IVith this arrangement and with coil C connected across im edance I, the potential impressed on coil will be 90 degrees out of phase with the motor currents, and the current in coil 'C 180 degrees out of phase with these currents that is, of the proper phase for producing a starting torque. As the motor starts, swltches 8 and 8' are thrown to their opposite positions, in order to produce the commutating field.

So far, the fluctuating field in theshortcircuited coils has been considered as due to the motor currents alone, which are by far the most important source of electro-motive force. But in all commutating machines, whether designed for direct or alternating current, there is another source of sparking at the brushes, which is termed commutation reactance. This consists in the induction due to the" rapid variation in the current flowing through the coil as it is commutated. The electro-motive force induced by the commutation reactance is in phase with the current. Although this cause of sparking is of much less importance in alternating current motors than the fluctuating field through the short-circuited coil, due to the motor currents, and may ordinarily be left out of consideration, it is possible, by means of my invention to completel neutralize this also. Thus, in Fig. 8 have shown two coils, one of which, C, is connected to the potential transformer P, while the other C, is in series with a coil short-circuited by the brush 6. The field of coil C will be practically 90 degrees out of phase with the motor currents and will act to neutralize the fluctuating field due to the motor currents, as has been heretofore explained; while the coil C will produce a field in phase with the motor currents, which will consequently neutralize commutation reactance. It is not necessary that separate poles should be used in order to produce the two field components to neutralize both effects.

1 A single coil, having impressed upon it an electro-motive force of the roper phase relation, will serve to neutralize the effect of all sources of electro-motive force in the short-circuited coils. The arrangements of Fi s. 5 or 6, or their equivalent, may be utiIized for obtaining the roper electromotive force to be impresse upon a sin 1e coil for neutralizing the fluctuatin fie (is due to both the motor currents and t e variation of current in the coil that is being commutated.

In Fig. 10 I have shown a series motor structure adapted to my invention. The rimary windings are indicated by the re er ence number 1, and are of the well known basket ty e of distributed Winding- The machine s own is a four-pole machine. In addition to the windings 1 I have shown auxiliary windings 2 displaced 90 degrees from the windin s 1. The purpose of winding 2 is to shift t e line of magnetization of the motor in order to reverse the direction of rotation, as described in-my application for United States patent, Serial No. 179,031, filed October 29, 1903. C C represent the commutating coils of my inventlon for the accommodation ofwhich certain of the slots for the primary winding are extended as shown in Fig.'11. In this figure, 3 represents a portion of a unching for the stator of the motor, vand s ows some of the slots extended as at 4 to receive the 'commutating coils. This construction makes a compact motor esigned to operate at practically obn stant speeds and loads, since in this case the commutating field. if once properly adjusted, will be at all times approximately correct. Furthermore. even in the case of a motor designed for operation at Various speeds and loads, control of the current in he commutator coils may be omitted if it is desired to simplify the arrangement. In this case, the electro-motire force may be adjusted for the average speed and load of the motor, and the commutation for all speeds and loads will be materially improved. Accordingly, I do not desire to limit myself to the particular construction and arrangement of parts here shown, since changes which do not do art from the spirit of my invention, and which are within the scope of the appended claims, will be obvious to those skilled in the art.

What I claim as new,and desire to secure by Letters Patent of the United States, is,--

1. In an alternating current. dynamo-electric machine having a fluctuating field, a rotor winding provided with a commutator, brushes bearing on said commutator, and means for inducing in the coils short-circuited by the brushes an electro-motive force equal and opposite to the electro-motive force due to the fluctuating field threading said short-circuited coils.

2. In an alternating current dynamo-electric machine havin a fluctuating field, a rotor winding provi ed with a commutator, brushes bearing on said commutator, and means for producing a local field ada ted to be cut by the coils short-circ'uited y the brushes and of a magnitude and phase adapted to produce in said coils an electromotive force equal and'opposite to the electro-motive force induced by thefluctuating field threading said coils.

3. In an alternating current dynamo-electric machine having a fluctuating field, a

rotor winding provided with a commutator,

brushes bearing on said commutator, and means for neutrallzmg at varying speeds and loads the 'electro-motlve force n the coils short-circuited by the brushes induced by the fluctuating field threading said coils.

4. In an alternating current dynamo-elect-ric machine having a fluctuating field,-a-

' motive force induced by the fluctuating field threading said coils.

5. In an alternating current motor, a rotor winding provided .with 'a commutator, brushes bearing on said commutator, a comon said commutator, a com-' coil a current v a ma nitude mutating coil adapted to produce a field to be cut by the rotor coils short-'circuited by the brushes, means for impressing an-electro-' motive force on said coil, and means for varying said electro-motive force as the load and speed of the motor vary.

(3. In an alternating current motor, a rotor winding provided with a commutator,

brushes bearing on said commutator, a commutating coil adapted to produce a field to be cut by the rotor coils short-circuited by the brushes, andmeans for impressin on said coll an electro-motive force in p ass with and varying with the currents supplied I to the motor.

' 7. In an alternating current motor, a rotor winding provided with a commutator, brushes bearing on said commutator, a commutating coil adapted to produce a field to be cut b the rotor coils short-circuited by the brusies, means for impressin on said coil an electro-motive force in pulse with and varying with the currents supplied to the motor, and means for varying the said electro-motive force as the motor speed varies.

8. In an alternating current motor, a rotor winding provided with a commutator, brushes bearing on said commutator, a commutating coil adapted to produce a field to be out b the rotor coils-short-circuited by the brushes, means for varying the voltage impressed upon the motor, and means for impressing on said commutating coll a voltage in phase with the fluctuatm field threading theshort-circuited coils an varying with the impressed motor voltage.

9. In an alternating current motor, a rotorwinding provided with a commutator, brushes bearing on said commutator, a commutating coil adapted to produce a field to be cut b the rotor coils short-circuited by the brus ies, means for varying the voltage impressed upon the motor, means for impressing on said commutating coil a voltage varying with the impressed motor voltage, and means for varying the voltage lmpressed onsaid coil as the motor speed Varies.

10. In an alternating current motor, a rotor winding provided with a commutator,

brushes bearing onlsaid commutator, a com mutating coil adapted to produce a field to be cut by the rotor coils short-errcurted by the brushes and thereby to neutralize the electro-motive force induced in said shortcircuited coils by thdQiffluctuating field threadin said coils, and m'eans for reversmg the relative direction of current flow through said coil at starting. I

11. In an alternating current motor, a rotor winding provided with a commutator,

brushes bearing-on said commutator, a commutating coil adapted to produce a field to be out ;b the rotor coils short-circuited by the brus es, means for impressing on said coil an electro-motive force in phase with the motor currents,'and means for shifting the phase of said electro-motive force at startm p 12. In an alternating current motor, a m

- means for shifting the phase of said electroto be cut by the rotor coils short-circuite motive force and reversing the connections of said commutating coil at starting.

18. In an alternating current motor, a rotor winding provided with a commutator, brushes bearing on said commutator, a commutating coil adapted to produce a field to be cut by the rotor coils short-circuited by the brushes and thereby to neutralize the electro-motive force induced in said shortcircuited coils by the fluctuating field threading them, and means for connecting said comniutating coil to increase the starting torque of the motor.

14. In an alternating current motor, a rotor winding provided with a commutator, brushes bearing on said commutator, means for producing a commutating field adaptgd Y the brushes and to produce in said coils an electro-motive force opposing the electromctive force induced by the fluctuating field threading said coils, and means for varying the phase of the commutating field at starting to increase the torque of the motor.

15. In an alternating current motor, a rotor winding provided with a commutator, brushes bearin on said commutator, and means for producing a field ada ted to be cut by the coils short-circuite d by the brushes, said field being adapted inbase and magnitude to produce in said coi s an electro-motive force equal and o posite to the electro-motive forces induce in said coils while short-circuited.

16. In an alternating current motor, a rotor winding provided with a commutator, brushes bearin on said commutator, and means for pro ucin afield adapted to be cut by the coils s ort-circuited by the brushes, said field having one component .90 degrees out of base with the motor currents and of a magnitude to neutralize the electromotive force induced in saidcoils by. the

fluctuating field produced by the motor currents and a second com onent in phase with the motor currents an of a magnitude to neutralize the electro-motive force due to commutation reactance.

17. In an alternating current motor, a rotor winding provided with a commutator, a distributed stator windin arranged in slots on the inner periphery o the stator, a portion of the stator slots being extended out- I of the slots being extended radially away from the rotor to receive commutating coils therein.

19. In an alternating current dynamoelcctric machine having a fluctuating field, a rotor winding provided with a commutator, brushes bearing thereon, and a commutating coil disposed adjacent-to the rotor coils short-circuited by the brushes, said coil being supplied with current of proper phase and magnitude to produce a field which when cut by the short-circuited coils induces therein an electromotive force of opposite phase to the electromotive force induced by the fluctuating field threading said coil.

BO/The method ofimproving commutation in. alternating current dynamo-electric machines of the commutatortype having a fluctuating field, which consists in causing each coil as it is short-circuited by abrush to cut a field adapted in phase and magnitude to produce in the coil an electro-motive force equal and o osite to that induced by the fluctuating field threading the coil.

91. The method of improving commutation in alternating current dynamo-electric machines of the commutator type having a fluctuating field, which consists in producing a local field adjacent to the coils shortcircuited by a brush adapted in base and magnitude to produce in said coi s anelectro-motive force equal and o posite to that induced by the fluctuating eld threading said coils.

22'. The method of improving commuta- I tion in alternating currentmotors of the commutator type, which-consists in producing a local commutating field adapted to be cut by the coils short-circuited by a brush and to produce in said coils an electromotive force 0 posed to that induced by the fluctuating eld threadin said coils and var ing the magnitude of t e first named fiel as the load and s eed of the motor vary.

23; The method 0 improving commuta- -tion in alternating current motors of the commutator type, which consists in neutralizing at varylng speeds and loads the electro-motive force induced in the coils shortcircuited by a brush by the fluctuating field threading said coils.

'24. The met-hod of improving commutation in alternating current motors of the commutator type, which consists in producing a commutating field degrees out of phase with and'varying with the currents supplied to the motor.

25. The method of improving commuta- 1 tion in alternating current dynamo-electric machines of the commutator type having a fluctuating field, which consists in producing a commutating field 90 degrees out of phase with the fluctuating field threading said coils.

26. The method of improving commutation in alternating current dynan'io-electric machines of the commutator type having a fluctuating field, which consists in roduc: ing a field adapted to he cut by tie coils short-ci-rcuited by a brush and of phase and magnitude adapted to produce in said coils an electro-n'iotive force equal and opposite to that induced in said coils while shortcircuited.

27. In an alternating current motor of the commutator conduction type, the combination with the fluctuating main field fl-uctuating auxiliary connnutating fields and means for displacing the phase of the commutating field.

28. In an alternating currentmotor of the commutator conduction type, the combination with the fluctuating main field, of fluctuating auxiliary commutating fields disalternate current energizing said auxiliary fields.

2!). In an alternatmg motor of the commutator conduction type, the combination avith the main field, of auxiliary commutating fields disposed in the axis of brushes and means for regulating the phase and the strength of said auxiliary fields in relation to the main field.

30. In an alternating current motor of the commutator conduction type, the combination with the main field coils, of auxiliary commutating field coils disposed in the axis of brushes and of means in the circuit of the latter for regulating the phase and the strength of the current energizing said auxiliary field coils, substantially as described.

In witness whereof I have hereunto set my hand this 7th day of April, 1904.

MAURICE MILCH.

Vitnesses:

BENJAMIN B. HULL, HELEN Oaroan. 

